High tensile hot-rolled steel sheet having excellent strain aging hardening properties and method for producing the same

ABSTRACT

The present invention provides a high tensile strength hot-rolled steel sheet having superior strain aging hardenability, which has high formability and stable quality characteristics, and in which satisfactory strength is obtained when the steel sheet is formed into automotive components, thus enabling the reduction in weight of automobile bodies. Specifically, a method for producing a high tensile strength hot-rolled steel sheet having superior strain aging hardenability with a BH of 80 MPa or more, a ΔTS of 40 MPa or more, and a tensile strength of 440 MPa or more includes the steps of heating a steel slab to 1,000° C. or more, the steel slab containing, in percent by mass, 0.15% or less of C, 2.0% or less of Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or less of S, 0.02% or less of Al, 0.0050% to 0.0250% of N, and optionally 0.1% or less in total of at least one of more than 0.02% to 0.1% of Nb and more than 0.02% to 0.1% of V, the ratio N (mass %)/Al (mass %) being 0.3 or more; rough-rolling the steel slab to form a sheet bar; finish-rolling the sheet bar at a finishing temperature of 800° C. or more; cooling at a cooling rate of 20° C. to 40° C./s or more within 0.5 second after the finish-rolling; and coiling at a temperature of 650° C. to 450° C. or less.

TECHNICAL FIELD

[0001] The present invention relates to high tensile strength hot-rolledsteel sheets having superior strain aging hardenability. Moreparticularly, the invention relates to a high tensile strengthhot-rolled steel sheet having a TS (tensile strength) of 440 MPa ormore, and relates to a method for producing the same. The high tensilestrength hot-rolled steel sheet is mainly used for automobiles as a thinhot-rolled steel sheet having high workability. Furthermore, the hightensile strength hot-rolled steel sheet is-used as a replacement for athin cold-rolled steel sheet having a thickness of approximately 4.0 mmor less and which was employed because it was difficult to produce asteel sheet with such a small thickness by hot rolling. The applicationsof the steel sheet in accordance with the present invention extend overa wide range from use for relatively light working, such as slightbending and forming of pipes by roll forming, to relatively heavyworking, such as drawing by a press.

[0002] The present invention concerns not only hot-rolled steel sheetsbut also electroplated steel sheets and hot-dip plated steel sheetsusing the hot-rolled steel sheets as mother plates.

[0003] In the present invention, “having superior strain aginghardenability” means to have the following characteristics:

[0004] 1) when a steel sheet is subjected to predeformation with atensile strain of 5% and then aging treatment by retaining the steelsheet at 170° C. for 20 minutes, an increase in deformation stressbefore and after the aging treatment (hereinafter referred to as BH;BH=Yield stress after aging treatment—Predeformation stress before agingtreatment) is 80 MPa or more; and

[0005] 2) an increase in tensile strength before and after strain agingtreatment (the predeformation +the aging treatment) (herein afterreferred to as ΔTS; ΔTS=Tensile strength after aging treatment—Tensilestrength before predeformation) is 40 MPa or more.

BACKGROUND ART

[0006] Many thin steel sheets are used as materials for automobilebodies. Cold-rolled steel sheets used to be used for applications inwhich superior formability is required. However, owing to adjustment ofsteel compositions (chemical constituents) and optimization of hotrolling conditions, it has become possible to produce hot-rolled steelsheets having high formability (high workability), and therefore, thehot-rolled steel sheets are increasingly used as materials forautomobile bodies.

[0007] In order to meet restrictions on exhaust gas in view of theglobal environment, reductions in automobile body weight are veryimportant. In order to reduce the automobile body weight, it iseffective to increase the tensile strength of steel sheets and decreasethe thickness of the steel sheets. Automotive components to which highertensile strength and thinner steel sheets are applied must have variouscharacteristics. For example, the required characteristics includestatic strength to bending and torsional deformation, fatigue strength,and impact resistance. Therefore, the high tensile strength steel sheetsused for the automotive components must have such characteristics afterformation and working are performed.

[0008] On the other hand, press forming is performed to steel sheetswhen automotive components are manufactured. Excessively high strengthof the steel sheets gives rise to problems; for example, shapefixability is degraded, and defects, such as cracking and necking, arecaused during formation due to a decrease in ductility. Such problemshave hindered the expansion of the application of high tensile strengthsteel sheets to automobile bodies.

[0009] In order to overcome the difficulties described above, forexample, with respect to cold-rolled steel sheets for outer panels, asteel sheet production technique is known in which an ultra low carbonsteel is used as a raw material and the C amount ultimately remaining inthe dissolved state is restricted within an appropriate range. In thistechnique, a strain aging hardening phenomenon, which occurs in a paintbaking step performed at 170° C.×approximately 20 minutes after pressforming, is used. Shape fixability and ductility are secured duringformation by maintaining the softness, and dent resistance is securedafter formation by an increase in YS (yield stress) due to strain aginghardening. However, in this technique, in order to avoid stretcherstrain leading to surface defects, an amount of the increase in YScannot be increased sufficiently, and since ΔTS is as small as severalMpa, the thickness of the steel sheet cannot be decreased sufficiently.

[0010] On the other hand, in the applications in which appearance is nota great problem, a steel sheet in which the bake hardening amount isfurther increased by using dissolved N (Japanese Examined PatentApplication Publication No. 7-30408), and a steel sheet in which bakehardenability is further improved by using a dual-phase structurecomposed of ferrite and martensite (Japanese Examined Patent ApplicationPublication No. 8-23048) have been disclosed.

[0011] However, in such steel sheets, although a higher bake hardeningamount can be obtained because YS (yield stress) is increased to acertain extent after paint baking, it is not possible to increase TS(tensile strength), and no great improvement in fatigue resistance andimpact resistance after formation is expected. Therefore, the steelsheets cannot be used for components in which fatigue resistance, impactresistance, etc., are required, which is disadvantageous. Since theamount of the increase in the yield stress YS is unstable, it is notpossible to decrease the thickness of the steel sheets in such a way asto contribute to lightening of automotive components, which is alsodisadvantageous.

[0012] Moreover, when a thin steel sheet with a thickness of 2.0 mm orless is produced, since the shape of the steel sheet becomesunsatisfactory in the hot rolling process, it is considerably difficultto press-form the steel sheet.

[0013] It is an object of the present invention to provide a hightensile strength hot-rolled steel sheet having superior strain aginghardenability which overcomes the limitations of the conventionaltechniques described above, which has high formability and stablequality characteristics, and in which satisfactory strength is obtainedwhen the steel sheet is formed into automotive components, thus greatlycontributing to lightening of automobile bodies. It is another object ofthe present invention to provide a method for industrially producingsuch a steel sheet at low costs and without disturbing the shapethereof.

DISCLOSURE OF INVENTION

[0014] In order to solve the problems described above, the presentinventors have produced various steel sheets by changing compositionsand production methods and have conducted many material evaluationtests. As a result, it has been found that an improvement in formabilityand an increase in strength after formation are easily made compatiblewith each other by using N, which has not been used positively in thefield where high workability is required, as a strengthening element,and by effectively using a large strain aging hardening phenomenonexhibited by the action of N as the strengthening element. In order toeffectively use the strain aging hardening phenomenon by N, the strainaging hardening phenomenon by N must be effectively combined with paintbaking conditions for automobiles and heat-treating conditions afterformation. The present inventors have found that it is effective toadjust the microstructure and the amount of dissolved N in a steel sheetwithin predetermined ranges by optimizing the hot rolling conditions. Ithas also been found that in order to stably cause the strain aginghardening phenomenon by N, it is particularly important to control theAl content according to the N content in terms of compositions.

[0015] That is, by using N as the strengthening element, by adjustingthe content of Al which is a key element in an appropriate range, and byproperly setting the hot rolling conditions so that the-microstructureand the dissolved N are optimized, it is possible to obtain a steelsheet (steel sheet of the present invention) having significantlysuperior formability and strain aging hardenability compared to aconventional solid-solution strengthening type C-Mn steel sheet and aprecipitation strengthening steel sheet (conventional steel sheets).

[0016] In general, in order to evaluate bake hardenability, a tensiletest is used. Since large variations in strength occurred when theconventional steel sheets were subjected to plastic deformation underthe actual press conditions, the conventional steel sheets could not beapplied to components in which high reliability was required even if theconventional steel sheets were evaluated as having desired bakehardenability in the tensile test. In contrast, variations in strengthare small when the steel sheet of the present invention is subjected toplastic deformation under the actual press conditions. Furthermore, thesteel sheet of the present invention has a higher evaluation of bakehardenability according to the tensile test compare to the conventionalsteel sheets. It has been found that stable component strengthcharacteristics are obtained by using the steel sheet of the presentinvention.

[0017] The thin hot-rolled steel sheet used for automobile bodies musthave very accurate shape and dimension. It has been found that accuracyof shape and dimension is greatly improved by employing a continuousrolling technique which has recently been put into practical use in thehot rolling process for producing the steel sheet of the presentinvention. Furthermore, it has been found that variations in materialproperties can be greatly decreased by partially heating or cooling therolled material so that the temperature profiles in the width directionand in the lengthwise direction become uniform.

[0018] The present invention has been achieved based on the findingsdescribed above and are summarized as follows.

[0019] (1) A high tensile strength hot-rolled steel sheet havingsuperior strain aging hardenability contains, in percent by mass, 0.15%or less of C, 2.0% or less of Si, 3.0% or less of Mn, 0.08% or less ofP, 0.02% or less of S, 0.02% or less of Al, 0.0050% to 0.0250% of N, andthe balance being Fe and incidental impurities, the ratio N (mass %)/Al(mass %) being 0.3 or more, N in the dissolved state being 0.0010% ormore.

[0020] (2) A high tensile strength hot-rolled steel sheet havingsuperior strain aging hardenability with a tensile strength of 440 MPaor more contains, in percent by mass, 0.15% or less of C, 2.0% or lessof Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or less of S, 0.02%or less of Al, 0.0050% to 0.0250% of N, and the balance being Fe andincidental impurities, the ratio N (mass %)/Al (mass %) being 0.3 ormore, N in the dissolved state being 0.0010% or more, and also has astructure in which the areal rate of the ferrite phase having an averagegrain size of 10 μm or less is 50% or more.

[0021] (3) A steel sheet according to (2) further contains at least oneselected from the group consisting of the following Group a to Group d:

[0022] Group a: 1.0% or less in total of at least one of Cu, Ni, Cr, andMo

[0023] Group b: 0.1% or less in total of at least one of Nb, Ti, and V

[0024] Group c: 0.0030% or less of B

[0025] Group d: 0.0010% to 0.010% in total of at least one of Ca andREM.

[0026] (4) A steel sheet according to either (2) or (3), wherein thethickness of the high tensile strength hot-rolled sheet is 4.0 mm orless.

[0027] (5) A high tensile strength hot-rolled plated steel sheetproduced by electroplating or hot-dip plating a steel sheet according toany one of (2) to (4).

[0028] (6) A method for producing a high tensile strength hot-rolledsteel sheet having superior strain aging hardenability with a tensilestrength of 440 MPa or more includes the steps of heating a steel slabto 1,000° C. or more, the steel slab containing, in percent by mass,0.15% or less of C, 2.0% or less of Si, 3.0% or less of Mn, 0.08% orless of P, 0.02% or less of S, 0.02% or less of Al, 0.0050% to 0.0250%of N, and optionally further containing at least one selected from thegroup consisting of the following Group a to Group d, the ratio N (mass%)/Al (mass %) being 0.3 or more; rough-rolling the steel slab to form asheet bar; finish-rolling the sheet bar at a finishing temperature of800° C. or more; cooling at a cooling rate of 20° C./s or more within0.5 second after the finish-rolling; and coiling at a temperature of650° C. or less:

[0029] Group a: 1.0% or less in total of at least one of Cu, Ni, Cr, andMo

[0030] Group b: 0.1% or less in total of at least one of Nb, Ti, and V

[0031] Group c: 0.0030% or less of B

[0032] Group d: 0.0010% to 0.010% in total of at least one of Ca andREM.

[0033] (7) A method according to (6) further includes the step ofperforming at least one of skin pass rolling and leveling with anelongation of 1.5% to 10% after the coiling step is performed.

[0034] (8) A method according to either (6) or (7) further includes thestep of joining consecutive sheet bars to each other between the stepsof rough-rolling and finish-rolling.

[0035] (9) A method according to any one of (6) to (8) further includesthe step of using at least one of a sheet bar edge heater for heating awidthwise end of the sheet bar and a sheet bar heater for heating alengthwise end of the sheet bar between the steps of rough-rolling andfinish-rolling.

[0036] (10) A high tensile strength hot-rolled steel sheet havingsuperior strain aging hardenability with a BH of 80 MPa or more, a ΔTSof 40 MPa or more, and a tensile strength of 440 MPa or more contains,in percent by mass, 0.15% or less of C, 2.0% or less of Si, 3.0% or lessof Mn, 0.08% or less of P, 0.02% or less of S, 0.02% or less of Al,0.0050% to 0.0250% of N, and the balance being Fe and incidentalimpurities, the ratio N (mass %)/Al (mass %) being 0.3 or more, N in thedissolved state being 0.0010% or more, and also has a structure in whichthe areal rate of the ferrite phase having an average grain size of 10μm or less is 70% or more, and the areal rate of the martensite phase is5% or more.

[0037] (11) A method for producing a high tensile strength hot-rolledsteel sheet having superior strain aging hardenability with a BH of 80MPa or more, a ΔTS of 40 MPa or more, and a tensile strength of 440 MPaor more includes the steps of heating a steel slab to 1,000° C. or more,the steel slab containing, in percent by mass, 0.15% or less of C, 2.0%or less of Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or less ofS, 0.02% or less of Al, 0.0050% to 0.0250% of N, and optionally furthercontaining at least one selected from the group consisting of thefollowing Group a to Group d, the ratio N (mass %)/Al (mass %) being 0.3or more; rough-rolling the steel slab to form a sheet bar;finish-rolling the sheet bar at a finishing temperature of 800° C. ormore; cooling at a cooling rate of 20° C./s or more within 0.5 secondafter the finish-rolling; and coiling at a temperature of 450° C. orless:

[0038] Group a: 1.0% or less in total of at least one of Cu, Ni, Cr, andMo

[0039] Group b: 0.1% or less in total of at least one of Nb, Ti, and V

[0040] Group c: 0.0030% or less of B

[0041] Group d: 0.0010% to 0.010% in total of at least one of Ca andREM.

[0042] (12) A high tensile strength hot-rolled steel sheet havingsuperior strain aging hardenability contains, in percent by mass, 0.03%to 0.1% of C, 2.0% or less of Si, 1.0% to 3.0% of Mn, 0.08% or less ofP, 0.02% or less of S, 0.02% or less of Al, 0.0050% to 0.0250% of N,0.1% or less in total of at least one of more than 0.02% to 0.1% of Nband more than 0.02% to 0.1% of V, and the balance being Fe andincidental impurities, the ratio N (mass %)/Al (mass %) being 0.3 ormore, N in the dissolved state being 0.0010% or more, the total ofprecipitated Nb and precipitated V being 0.015% or more, and also has astructure in which the areal rate of the ferrite phase having an averagegrain size of 10 μm or less is 80% or more, and the average grain sizeof a precipitate composed of a Nb carbonitride or a V carbonitride is0.05 μm or less.

[0043] (13) A method for producing a high tensile strength hot-rolledsteel sheet having superior strain aging hardenability includes thesteps of heating a steel slab to 1,100° C. or more, the steel slabcontaining, in percent by mass, 0.03% to 0.1% of C, 2.0% or less of Si,1.0% to 3.0% of Mn, 0.08% or less of P, 0.02% or less of S, 0.02% orless of Al, 0.0050% to 0.0250% of N, 0.1% or less in total of at leastone of more than 0.02% to 0.1% of Nb and more than 0.02% to 0.1% of V,and the balance being Fe and incidental impurities; rough-rolling thesteel slab to form a sheet bar; finish-rolling the sheet bar at afinishing temperature of 800° C. or more; cooling at a cooling rate of40° C./s or more within 0.5 second after the finish-rolling; and coilingin the temperature range of 550 to 650° C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044]FIG. 1 is a graph which shows BH (an increase in deformationstress) with respect to examples of the present invention andcomparative examples.

[0045]FIG. 2 is a graph which shows ΔTS (an increase in tensilestrength) with respect to examples of the present invention andcomparative examples.

BEST MODE FOR CARRYING OUT THE INVENTION

[0046] First, the chemical compositions of steel in the presentinvention will be described. The content (%) of each constituent elementis shown in percent by mass.

[0047] C: 0.15% or less

[0048] C is an element which increases the strength of steel sheets, andin order to ensure desired strength, the C content is preferably set at0.005% or more. The C content is also preferably set at 0.005% or morein order to suppress grain coarsening. If the C content exceeds 0.15%,the following problems arise. (1) Since the percentage of carbides insteel becomes excessive and the ductility of steel sheets is greatlydecreased, formability is degraded. (2) Spot weldability and arcweldability are greatly degraded. (3) With respect to hot rolling of asteel sheet with a large width and a small thickness, deformationresistance greatly increases below the austenite low temperature range,and the rolling force rises suddenly, resulting in a difficulty inrolling. Therefore, the C content is set at 0.15% or less. Additionally,in view of an improvement in formability, the C content is preferably0.08% or less, and in applications where good ductility is particularlyimportant, the C content is more preferably 0.05% or less.

[0049] However, with respect to a steel sheet of the present inventioncontaining 0.1% or less in total of at least one of more than 0.02% to0.1% of Nb and more than 0.02% to 0.1% of V, the C content is preferablyset at 0.03% to 0.1%. C is an element which increases the strength ofsteel sheets and ensures desired strength by formation of carbonitrideswith Nb and V (precipitates), and thus the C content is preferably setat 0.03% or more. In order to suppress grain coarsening, preferably, theC content is also set at 0.03% or more. On the other hand, as will bedescribed below, in order to finely precipitate carbonitrides of Nb andV, after hot rolling is completed, the carbonitrides must beprecipitated in the low-temperature ferrite phase. If the C contentexceeds 0.1% at this stage, coarse carbonitrides are formed during hotrolling, resulting in a decrease in the strength of the steel sheet.Therefore, the C content is set at 0.1% or less.

[0050] Si: 2.0% or less

[0051] Si is an effective element which increases the strength of steelsheets without greatly decreasing the ductility of steel. On the otherhand, since Si greatly increases the Ar₃ transformation temperature, alarge amount of the ferrite phase tends to be generated during finishrolling. Si also adversely affects steel sheets, for example, degradingof surface properties and glossy surface. In order to obtain thestrength-increasing effect significantly, the Si content is preferablyset at 0.1% or more. If the Si content is 2.0% or less, it is possibleto inhibit a large increase of the transformation temperature byadjusting the amount of Mn which is added to steel in combination withSi, and satisfactory surface properties are also ensured. Therefore, theSi content is set at 2.0% or less. Additionally, in order to ensure highductility with a TS of more than 500 MPa, in view of the balance betweenstrength and ductility, the Si content is preferably set at 0.3% ormore.

[0052] Mn: 3.0% or less

[0053] Mn decreases the Ar₃ transformation temperature, and it ispossible to make Mn counter the action of Si for increasing thetransformation temperature. Mn is an element which is effective inpreventing hot brittleness due to S, and in view of preventing hotbrittleness, Mn is preferably added according to the amount of S. SinceMn has a grain refining effect, it is desirable that Mn be activelyadded so that Mn is used for improving material properties. In view ofstably fixing S, the Mn content is preferably set at approximately 0.2%or more, and in order to meet the strength requirement of TS 500 MPaclass, the Mn content is preferably set at 1.2% or more, and morepreferably, at 1.5% or more. By increasing the Mn content to such alevel, variations of mechanical properties and strain aginghardenability of steel sheets are reduced with respect to the change inhot rolling conditions, thus being effective in stabilizing the quality.

[0054] However, if the Mn content exceeds 3.0%, the following problemsarise. (1) Although the detailed mechanism is unknown, the deformationresistance at elevated temperatures of steel sheets tends to beincreased. (2) Weldability and formability at the welding zone tend tobe degraded. (3) Since the generation of ferrite is greatly suppressed,ductility is degraded. Therefore, the Mn content is preferably limitedto 3.0% or less. Additionally, in applications where more satisfactorycorrosion resistance and formability are required, the Mn content ispreferably set at 2.5% or less.

[0055] With respect to a product with particularly small thickness,since the quality and shape are minutely changed due to the variation ofthe transformation temperature, it is important to more strictly balancebetween the action of Mn for decreasing the transformation temperatureand the action of Si for increasing the transformation temperature. Fromsuch a viewpoint, in the steel sheet used for automobile bodies with athickness of approximately 4.0 mm or less, the ratio Mn/Si (ratiobetween the Mn amount and the Si amount) is preferably set at 3 or more.

[0056] However, with respect to a steel sheet of the present inventioncontaining 0.1% or less in total of at least one of more than 0.02% to0.1% of Nb and more than 0.02% to 0.1% of V, the Mn content ispreferably set at 1.0% to 3.0%. If the Mn content is less than 1.0%, theAr₃ transformation temperature increases, and carbonitrides areremarkably formed in the high-temperature ferrite phase, and since thecarbonitrides coarsen, it becomes difficult to ensure desired strength.Therefore, the Mn content must be 1.0% or more.

[0057] P: 0.08% or less

[0058] Although P is effective as a solid-solution strengtheningelement, if the P content is excessive, steel is embrittled and thestretch-flanging property of the steel sheet is degraded. P also tendsto segregate in steel, resulting in embrittlement at the welding zone.Therefore, the P content is set at 0.08% or less. Additionally, when thestretch-flanging property and toughness at the welding zone are regardedas particularly important, the P content is preferably set at 0.04% orless.

[0059] S: 0.02% or less

[0060] S is an element which is present as an inclusion, degrades theductility of the steel sheet, and also degrades the corrosionresistance. Therefore, the S content is limited to 0.02% or less. Inapplications where particularly good workability is required, the Scontent is preferably set at 0.015%. When the required level of thestretch-flanging property, which is particularly susceptible to the Samount, is high, the S content is preferably 0.008% or less. Althoughthe detailed mechanism is unknown, if the S content is decreased to0.008% or less, the strain aging hardenability of the hot-rolled steelsheet tends to be stabilized at a higher level. For this reason, the Scontent is also preferably 0.008% or less.

[0061] Al: 0.02% or less

[0062] Al is added to steel as a deoxidizing element, which is effectivein improving the cleanness of the steel, and Al is also preferably addedto the steel in order to achieve texture refinement. However, if the Alcontent is excessive, the following problems arise. (1) The surfaceproperties of steel sheets are degraded. (2) The amount of dissolved Nwhich is important in the present invention is decreased. (3) Even ifdissolved N is ensured, if the Al content exceeds 0.02%, variations instrain aging hardenability due to the change in production conditionsare increased. Therefore, the Al content is limited to 0.02% or less.Additionally, in view of material stability, the Al content is morepreferably set at 0.001% to 0.016%.

[0063] N: 0.0050% to 0.0250%

[0064] N is the most important constituent element in the presentinvention. That is, by the addition of an appropriate amount of N tocontrol the production conditions, it is possible to secure a necessaryand sufficient amount of N in the dissolved state in the mother plate(as hot rolled). Thereby, the effect of an increase in strength (YS, TS)due to solid-solution strengthening and strain aging hardening issatisfactorily exhibited, and it is possible to stably satisfy themechanical property conditions of the steel sheet of the presentinvention, i.e., TS of 440 MPa or more, BH of 80 MPa ore more, and ΔTSof 40 MPa or more. N also decreases the Ar₃ transformation temperature.Since it is possible to prevent a thin steel sheet, whose temperature iseasily decreased during hot rolling, from being rolled at a temperaturelower than the Ar₃ transformation temperature, N is effective instabilizing operation.

[0065] If the N content is less than 0.0050%, it is not possible toobtain the strength-increasing effect. On the other hand, if the Ncontent exceeds 0.0250%, the rate of occurrence of internal defects ofthe steel sheet increases, and also slab cracking during continuouscasting, etc., often occurs. Therefore, the N content is set at 0.0050%to 0.0250%. In view of material stability and improvements in yield inconsideration of the whole manufacturing process, the N content ispreferably set at 0.0070% to 0.0170%. Additionally, if the N content isin the range of the present invention, there are no adverse effects onweldability.

[0066] Even if N is added, if the N content is in the range of thepresent invention, there is substantially no increase in deformationresistance at elevated temperatures during the production of steelsheets. It has been found that use of strengthening due to N issignificantly advantageous to the production of high tensile strengththin hot-rolled steel sheets.

[0067] N in the dissolved state: 0.0010% or more

[0068] In order to ensure sufficient strength in the mother plate and toexhibit satisfactory strain aging hardenability due to N, i.e., to setthe BH at 80 MPa or more and the ΔTS at 40 MPa or more, 0.0010% or moreof N in the dissolved state (hereinafter referred to as “dissolved N”)must be present in steel. Herein, the amount of dissolved N is found bysubtracting the amount of precipitated N from the total amount of N insteel. As a method for extracting precipitated N, i.e., as a method fordissolving ferrite, an acidolysis, a halogen process, or an electrolyticprocess may be used. As a result of comparative study among thesemethods for dissolving ferrite, the present inventors have found thatthe electrolytic process is most superior. In the electrolytic process,only ferrite can be stably dissolved without decomposing significantlyunstable precipitates, such as carbides and nitrides. Accordingly, inthe present invention, precipitated N is extracted by dissolving ferriteusing the electrolytic process. As an electrolytic solution, anacetylacetone-based solution is used, and electrolysis is performed at aconstant potential. The residue extracted by the electrolytic process ischemically analyzed to find the N amount in the residue, which isdefined as the amount of precipitated N.

[0069] Additionally, in order to achieve large BH and ΔTS, the amount ofdissolved N is preferably set at 0.0020% or more, and in order toachieve larger BH and ΔTS, the amount of dissolved N is preferably setat 0.0030% or more.

[0070] N/Al (ratio between the N amount and the Al amount): 0.3 or more

[0071] As described above, in order to keep 0.0010% or more of dissolvedN stably without being affected by the production conditions, the amountof Al, which is an element for strongly fixing N, must be limited to0.02% or less. As a result of searching for the conditions in which theamount of dissolved N after hot rolling is 0.0010% or more with respectto steels in which the combination of the N amount and the Al amount iswidely changed within the compositional range of the present invention,it has been found that the ratio N/Al must be 0.3 or more. Furthermore,cooling conditions and the coiling temperature condition afterfinish-rolling must be set in the ranges described below. Therefore, theAl amount is limited to N/0.3 or less.

[0072] Group a: 1.0% or less in total of at least one of Cu, Ni, Cr, andMo

[0073] Since all of the elements Cu, Ni, Cr, and Mo in Group acontribute to an increase in the strength of steel sheets, they may beadded alone or in combination. However, if it is an excessive amount,deformation resistance at elevated temperatures is increased, chemicalconversion properties and surface treatment properties in a broad senseare degraded, formability at the welding zone is degraded due tohardening of the welding zone, and so on. Therefore, the total amount ofGroup a is preferably 1.0% or less.

[0074] Group b: 0.1% or less in total of Nb, Ti, and V

[0075] Since all of the elements Nb, Ti, and V in Group b contribute torefinement and uniformization of the grain size, they may be added aloneor in combination. However, if the amount is excessive, deformationresistance at elevated temperatures is increased, chemical conversionproperties and surface treatment properties in a broad sense, such aspaintability, are degraded, formability at the welding zone is degradeddue to hardening of the welding zone, and so on. Therefore, the totalamount of Group b is preferably 0.1% or less.

[0076] Group c: 0.0030% or less of B

[0077] The element B in Group c improve the hardenability of steel. B isappropriately added to steel in order to increase the strength of thesteel by changing the structure phases other than ferrite tolow-temperature transformation phases. However, if the amount isexcessive, since B precipitates as BN, it is not possible to secure thedissolved N. Therefore, the B content must be limited to 0.0030% orless.

[0078] Group d: 0.0010% to 0.010% in total of at least one of Ca and REM

[0079] The elements Ca and REM in Group d control the shapes ofinclusions, and, in particular, when the stretch-flanging property isrequired, they are added alone or in combination. In such a case, if thetotal amount is less than 0.0010%, the control effect is insufficient.On the other hand, if the total amount exceeds 0.010%, the occurrence ofsurface defects becomes conspicuous. Therefore, the total amount ofGroup d to be added is preferably set in the range of 0.0010% to 0.010%.

[0080] When Nb and V are added in the present invention, preferably,0.1% in total of at least one of more than 0.02% to 0.1% of Nb and morethan 0.02% to 0.1% of V is contained.

[0081] Nb and V are important constituent elements in the presentinvention. By adding appropriate amounts of Nb and V and by controllingthe production conditions as described below, it is possible to form anappropriate amount of significantly fine carbonitrides, and desiredstrength is ensured and the yield ratio can be greatly increased.Thereby, fatigue resistance and impact resistance are remarkablyimproved. Furthermore, the fine carbonitrides of Nb and V improve thestrain aging hardenability and contribute to refinement anduniformization of the ferrite grain size. If the content of Nb or V(i.e., the concentration of the additive constituent in steel) is 0.02%or less, the effect thereof is small, and therefore, the content of Nbor V is set at more than 0.02%.

[0082] On the other hand, the content of Nb and V (total content whenboth elements are added in combination) exceeding 0.1% gives rise toproblems; for example, (1) an increase in deformation resistance atelevated temperatures, (2) degradation of chemical conversion propertiesand surface treatment properties, such as paintability, and (3)degradation of formability at the welding zone due to hardening at thewelding zone. Therefore, the content of Nb and V (total content whenboth elements are added in combination) is set at 0.1% or less.

[0083] Total amount of precipitated Nb and precipitated V: 0.015% ormore

[0084] Nb and V are precipitated as fine carbonitrides, thus increasingstrength and improving strain aging hardenability. If the amount of Nbor V present as carbonitrides, or the total amount of these when Nb andV are added in combination, is less than 0.015%, the strength increasingeffect and the strain aging hardenability improving effect are notexhibited sufficiently. In the composition of steel of the presentinvention, since substantially all the precipitation of Nb and V areprecipitated as carbonitrides, the amount of Nb and the amount of Vpresent as carbonitrides of Nb and V are determined by measuring theamount of precipitated Nb and the amount of precipitated V,respectively. Therefore, the total amount of precipitated Nb andprecipitated V is limited to 0.015% or more. Herein, in order to measurethe amount of precipitated Nb and the amount of precipitated V,extraction is performed by the electrolysis process described above, andthe amount of Nb and the amount of V in the residue are determined asprecipitated Nb and precipitated V.

[0085] Next, the structure and mechanical properties of steel sheetswill be described.

[0086] Areal Rate of Ferrite Phase:

[0087] Steel sheets used for automobiles must have satisfactoryworkability. In order to ensure ductility necessary as steel sheets usedfor automobiles, the areal rate of the ferrite phase is preferably 50%or more.

[0088] Additionally, when high strength is required, the areal rate ofthe ferrite phase is set at less than 50%, and the areal rate of thebainite phase or the martensite phase is set at 35% or more, or thetotal areal rate thereof is set at 35% or more. By using such astructural composition, the steel sheet having a tensile strength of 780Mpa or more, as steel sheet tensile characteristics, is easily obtained.In such a case, the steel sheet is preferably applied to a section inwhich an emphasis is placed on strength rather than on ductility in theautomotive application.

[0089] When satisfactory ductility is required, the areal rate of theferrite phase is preferably set at 70% or more, and when moresatisfactory ductility is required, the areal rate of the ferrite phaseis more preferably set at 80% or more. Herein, examples of ferrite alsoinclude bainitic ferrite and acicular ferrite which do not containcarbides, in addition to so-called ferrite (polygonal ferrite).

[0090] Additionally, although phases other than the ferrite phase arenot specifically limited, in view of increasing strength, each singlephase of bainite, martensite, and retained austenite or a mixed phasethereof is preferred.

[0091] Average Grain Size of Ferrite Phase: 10 μm or less

[0092] In the present invention, the average grain size is determined bythe value which is larger when compared between the value measured bymensuration according to ASTM based on a photograph of the sectionalstructure and the nominal grain size measured by an intercept method(for example, refer to “Thermal Treatment” 24 (1984) 334 by Umemoto, etal.).

[0093] In the present invention, although dissolved N is secured in themother plate, according to the experiment and analysis results by thepresent inventors, even if the amount of dissolved N is kept at acertain level, if the average grain size of the ferrite phase exceeds 10μm, variations in strain aging hardenability are increased. Although thedetailed mechanism for the above is unknown, the segregation andprecipitation of alloying elements in the grain boundaries, and workingand heat treatment applied thereto are considered to be related to thevariations. Independent of the reasons, in order to stabilize strainaging hardenability, the average grain size of the ferrite phase must beset at 10 μm or less. Additionally, in order to further improve andstabilize BH and ΔTS, the average grain size is preferably set at 8 μmor less.

[0094] When the martensite phase (M phase) is contained in the structurein the present invention, the areal rate of the M phase is preferably 5%or more. The M phase contained in the structure at the areal rate of 5%or more is effective in the present invention. Thereby, the steel sheethas satisfactory ductility in spite of high strength and high BH andΔTS. If the areal rate of the M phase is less than 5%, the effectthereof is not obtained sufficiently. Due to the presence of themartensite phase at the areal rate of 5% or more, in addition to theimprovement in ductility, the yield ratio =YS/TS is decreased, and theshape fixability improving effect is remarkably exhibited particularlywhen working is performed in the minute strain range.

[0095] In view of ductility and the low yield ratio, the areal rate ofthe M phase is preferably less than 35%, and more preferably, 7% to 20%.In such a case, in the steel sheet of the present invention, in additionto ferrite and martensite, the bainite phase, the pearlite phase, etc.,may be contained in the structure if the areal rate thereof is severalpercent.

[0096] On the other hand, in view of an increase in strength, the arealrate of the M phase is preferably 35% or more, or the total area rate ofthe M phase and the bainite phase is preferably 35% or more. In such acase, the structure may contain the pearlite phase and the retainedaustenite phase at the areal rate of several percent, in addition to theferrite, bainite, and martensite phases.

[0097] In the present invention, when Nb and V are added, the averagegrain size of the precipitate comprising Nb or V carbonitrides ispreferably 0.05 μm or less. In order for the carbonitrides of Nb or V toincrease strength and to improve strain aging hardenability, thecarbonitrides must be precipitated finely. If the average grain size ofthe carbonitrides is coarser than 0.05 μm, the effects thereof are notexhibited. Therefore, the average grain size of the carbonitrides is setat 0.05 μm or less.

[0098] Additionally, in order to measure the grain size of thecarbonitrides of Nb and V, at least 20 visual fields are observed by atransmission electron microscope with a magnifying power of 100,000using thin films. With respect to the precipitates observed,carbonitrides of Nb and V are identified using an energy-dispersiveX-ray analyzer (EDX). The grain size is defined as ½ of the sum of thedetermined breadth and length of the carbonitride of Nb and V. The grainsize is measured for all the carbonitrides of Nb and V in the visualfield, and the average of the total sum is defined as the average grainsize.

[0099] Tensile Strength (TS): 440 MPa or more

[0100] A steel sheet used for structural members of automobile bodiesmust have a TS of 440 MPa or more. A steel sheet used for structuralmembers in which further strength is required must have a TS of 540 MPaor more.

[0101] Strain Aging Hardenability

[0102] In the present invention, as described above, “having superiorstrain aging hardenability” means to have the following characteristics:

[0103] 1) when a steel sheet is subjected to predeformation with atensile strain of 5% and then aging treatment by retaining the steelsheet at 170° C. for 20 minutes, an increase in deformation stressbefore and after the aging treatment (hereinafter referred to as BH;BH=Yield stress after aging treatment—Predeformation stress before agingtreatment) is 80 MPa or more; and

[0104] 2) an increase in tensile strength before and after strain agingtreatment (the predeformation +the aging treatment) (herein afterreferred to as ΔTS; ΔTS Tensile strength after aging treatment—Tensilestrength before predeformation) is 40 MPa or more.

[0105] Predeformation with a Tensile Strain of 5%

[0106] When strain aging hardenability is defined, a prestrain(predeformation) is an important factor. The present inventors havestudied the influence of the prestrain on strain aging hardenability,assuming the deformation mode applied to steel sheets used forautomobiles. As a result, it has been found that (1) the deformationstress in the deformation mode described above can be substantiallyintegrated into a uniaxial stress (tensile strain) except for extremelydeep drawing; (2) in a real component, the uniaxial stress generallyexceeds 5%; and (3) component strength (strength of a real component)well corresponds to the strength obtained after strain aging treatmentwith a prestrain of 5% is performed. Based on the knowledge describedabove, the predeformation for the strain aging treatment is defined as atensile strain of 5%.

[0107] Aging Treatment Conditions: (Heating Temperature) 170°C.×(Retention Time) 20 minutes

[0108] In the conventional paint baking treatment conditions, 170° C.×20minutes is adopted as the standard. Therefore, 170° C.×20 minutes isdefined as the aging treatment conditions. Additionally, when a strainof 5% or more is applied to a steel sheet of the present inventioncontaining a large amount of dissolved N, hardening is performed bytreatment at a lower temperature. In other words, the aging conditionsmay be set more widely. In general, in order to increase the amount ofhardening, retention at a higher temperature for a longer time isadvantageous as long as softening is prevented.

[0109] Specifically, in the steel sheet of the present invention, thelower limit of the heating temperature in which hardening is noticeableafter predeformation is approximately 100° C. On the other hand, if theheating temperature exceeds 300° C., hardening hits the peak, and if theheating temperature is 400° C. or more, a tendency toward slightlysoftening appears, and also thermal strain and temper color becomeconspicuous. As for the retention time, hardening is satisfactorilyachieved if the retention time is set at approximately 30 seconds at aheating temperature of approximately 200° C. In order to achieve thelarger amount of hardening and stable hardening, the retention time ispreferably set at 60 seconds or more. However, even if retention isperformed for more than 20 minutes, no further hardening is achieved,and production efficiency is reduced, resulting in no practicalbenefits.

[0110] For the reasons described above, when the steel sheet of thepresent invention is used, after working is performed, preferably, theheating temperature is set at 100 to 300° C. and the retention time isset at 30 seconds to 20 minutes as the aging treatment conditions. Inthe present invention, even under the aging conditions oflow-temperature heating and short-time retention in which sufficienthardening is not achieved in the conventional paint baking type steelsheet, a large amount of hardening can be obtained. Additionally, themethod for heating is not specifically limited, and in addition toatmospheric heating using a furnace which is employed for general paintbaking, induction heating, heating by non-oxidizing flame, laser beam,or plasma, or the like may be preferably used.

[0111] H: 80 MPa or more, ΔTS: 40 MPa or more

[0112] Automobile components must have strength which can cope withcomplex stress loading from outside. Therefore, it is important for thematerial steel sheet to have a strength characteristic in the smallstrain range as well as a strength characteristic in the large strainrange. From this viewpoint, the present inventors have limited BH to 80MPa or more and TS to 40 MPa or more with respect to the steel sheet ofthe present invention to be used as a material for automobilecomponents. More preferably, BH is set at 100 MPa or more and ΔTS is setat 50 MPa or more. It is to be understood that the above limitationsdefine BH and ΔTS under the conditions of aging treatment of 170° C.×20minutes after a prestrain of 5% is applied. BH and ΔTS may be increasedalso by setting the heating temperature higher and/or by setting theretention time longer.

[0113] In the steel sheet of the present invention, even if acceleratedaging by heating (artificial heating) is not performed after forming andworking, only by leaving the steel sheet at room temperature, anincrease in strength corresponding to at least approximately 40% of fullaging is expected. Moreover, on the other hand, in the state in whichforming and working are not performed, even if the steel sheet is leftat room temperature for a long time, aging degradation, i.e., aphenomenon in which YS increases and El (elongation) decreases, does notoccur, which is a superior characteristic not observed in the known art.

[0114] When the thickness of the produced steel sheet exceeds 4.0 mm,the advantages of the present invention are lost because even theconventional steel sheet having large deformation resistance at elevatedtemperatures can be easily hot-rolled and because steel sheets having athickness of more than 4.0 mm are not substantially used forautomobiles. Therefore, the steel sheet of the present inventionpreferably has a thickness of 4.0 mm or less.

[0115] A plated steel sheet obtained by electroplating or hot-dipplating the steel sheet of the present invention also has TS, BH, andΔTS which are substantially the same as those before plating. As thetype of plating, any one of electro-galvanizing, hot-dip galvanizing,hot-dip galvannealing, electrotinning, electrolytic chromium plating,and electrolytic nickel plating may be preferably used.

[0116] Next, the method for producing the steel sheet of the presentinvention will be described.

[0117] The steel sheet of the present invention is produced basically bya hot-rolling process in which a steel slab having the compositionwithin the ranges of the present invention is heated, the steel slab isrough-rolled to form a sheet bar, the sheet bar is finish-rolled, andcoiling is performed after cooling. Although the slab is preferablyformed by continuous casting in order to avoid macroscopic segregationof constituents, the slab may be formed by an ingot-making method, or athin slab continuous casting method. Instead of the ordinary process inwhich the produced slab is cooled to room temperature and heating isperformed again, an energy-saving process, such as a process in which ahot slab without cooling is inserted into a furnace or a direct rollingprocess in which a produced slab is directly rolled after slightretention of heat, may be used. In particular, in order to efficientlysecure N in the dissolved state, direct rolling is one of the effectivetechniques.

[0118] Hot-rolling conditions are defined as follows.

[0119] Slab Heating Temperature: 1,000° C. or more

[0120] In order to secure the initial amount of dissolved N and to meetthe target (0.0010% or more) of dissolved N in the product, the slabheating temperature (hereinafter referred to as “SRT”) is set at 1,000°C. or more. Additionally, in order to avoid an increase in loss due tooxidation weight gain, the SRT is preferably 1,280° C. or less.Rough-rolling of the heated slab may be performed in a known method.

[0121] After rough-rolling is performed, the sheet bar is subjected tofinish-rolling. In the present invention, finish-rolling is preferablyperformed continuously by joining consecutive sheet bars to each otherbetween rough-rolling and finish-rolling. As the joining means,fusion-pressure welding, laser beam welding, electron beam welding, orthe like may be appropriately used.

[0122] Thereby, the proportion of non-steady sections (front ends andback ends of the processed member) in which the shape is easilydisturbed during finish-rolling and subsequent cooling is decreased, andthe stable rolling length (the continuous length which can be rolledunder the same conditions) and the stable cooling length (the continuouslength which can be cooled under tension) are extended, and therebyaccuracy of shape and dimension and the yield of the product areimproved. Lubrication-rolling, which was difficult to perform due tostability in continuous rolling and biting properties in theconventional single-shot rolling for each sheet bar, can be easilyperformed to thin, wide sheet bars, and the rolling force and thebearing stress are reduced, resulting in an extension of the rollerlife.

[0123] In the present invention, preferably, at least one of a sheet baredge heater for heating a widthwise end of the sheet bar and a sheet barheater for heating a lengthwise end of the sheet bar is used between thesteps of rough-rolling and finish-rolling so that the temperatureprofiles in the width direction and in the lengthwise direction becomeuniform. Thereby, the variations in material properties within the steelsheet can be further decreased. A sheet bar edge heater or sheet barheater of induction heating type is preferably used.

[0124] First, the temperature variation in the width direction iscompensated for by the sheet bar edge heater. At this stage, heating ispreferably adjusted so that the temperature range in the width directionat the finishing side in finish-rolling is within approximately 20° C.,although it depends on the steel composition, etc. Next, the temperaturevariation in the longitudinal direction is compensated for by the sheetbar heater. At this stage, heating is preferably adjusted so that thetemperature in the lengthwise end is higher than the temperature in thecenter by approximately 20° C.

[0125] Finishing Temperature in Finish-rolling: 800° C. or more

[0126] In finish-rolling, in order to adjust the texture of the steelsheet uniformly and finely, the finishing temperature in finish-rolling(hereinafter referred to as “FDT”) is set at 800° C. or more. If the FDTis less than 800° C., the finish-rolling temperature is too low and thetexture becomes nonuniform, and deformation textures partially remain,which may result in various problems during press forming. Although theremaining of such deformation textures may be avoided byhigh-temperature coiling, if high-temperature coiling is performed,coarse grains are generated and strength is decreased, and also theamount of dissolved N is also greatly decreased. Therefore, it becomesdifficult to obtain a target TS of 440 MPa. Additionally, in order tofurther improve the mechanical properties, the FDT is preferably set at820° C. or more.

[0127] In finish-rolling, to perform lubrication-rolling to reduce theload during hot-rolling is effective in uniformizing the shape andmaterial properties. In such a case, the coefficient of friction ispreferably in the range of 0.25 to 0.10, and it is desirable that thelubrication-rolling be performed in combination with the continuousrolling in view of the operational stability in hot-rolling.

[0128] Cooling after Rolling: Water-cooling at a cooling rate of 20°C./s or more started within 0.5 second after rolling

[0129] After rolling is completed, cooling is started immediately(within approximately 0.5 second), and the cooling must be performedrapidly at an average cooling rate of 20° C./s or more. If theseconditions are not satisfied, since grains grow excessively, refinementof the grain size is not achieved, and also, since AlN precipitatesexcessively due to strain energy introduced by rolling, the amount ofdissolved N becomes insufficient. Additionally, in order to ensureuniformity in the material properties and shape, the average coolingrate is preferably set at 300° C./s or less.

[0130] In the present invention, with respect to the cooling patternwhen the M phase is contained in the structure at the areal rate of 5%or more, cooling may be performed continuously as is usually done, or inorder to control the γ to α transformation during cooling and to achievethe phase separation in the structure advantageously, it is alsoeffective to perform slow cooling (interruption of rapid cooling) forapproximately 1 to 5 seconds at a rate of 10° C./s or less in thetemperature range of 700 to 800° C. However, after the slow cooling,rapid cooling must be performed again at a rate of 20° C./s or more.

[0131] Coiling Temperature: 650° C. or less

[0132] As the coiling temperature (hereinafter referred to as “CT”)decreases, the strength of the steel sheet increases, and in order toachieve the target TS of 440 MPa or more at CT 650° C. or less, the CTis set at 650° C. or less. Additionally, if the CT is less than 200° C.,the shape of the steel sheet is easily disturbed and problems may arisein practical use, and therefore, CT is preferably 200° C. or more. Inview of material uniformity, CT is preferably 300° C. or more, and morepreferably, more than 450° C.

[0133] In the present invention, when the M phase is contained in thestructure at the areal rate of 5% or more, the coiling temperature ispreferably set at 450° C. or less. The strength of the steel sheetincreases as the coiling temperature decreases. At a CT of 450° C. orless, the texture is refined and the areal rate of the M phase reaches5% or more, and thereby the target TS of 440 MPa or more is achieved.Therefore, the CT is set at 450° C. or less. Furthermore, in order toobtain the M phase stably, 40° C./s or more is preferable. Additionally,if the CT is less than 100° C., the shape of the steel sheet is easilydisturbed and the possibility of causing problems in practical useincreases. Therefore, the CT is preferably 100° C. or more. In view ofmaterial uniformity, the CT is preferably 150° C. or more.

[0134] In the present invention, when Nb and V are contained, thecoiling temperature is preferably set at 550 to 650° C. In such a case,if the coiling temperature is higher than 650° C., since carbonitridesof Nb and V are coarsened, it becomes difficult to adjust the grain sizethereof to 0.05 μm or less and the strength of the steel sheet is alsodecreased. If the CT is lower than 550° C., since precipitation ofcarbonitrides of Nb and V is suppressed, the predetermined amount ofcarbonitrides cannot be secured. Therefore, the CT is set at 550 to 650°C.

[0135] Furthermore, in the present invention, preferably, working(working after hot-rolling) is performed by at least one of skin passrolling and leveling with an elongation of 1.5% to 10% after coiling isperformed. Additionally, the elongation of skin pass rolling is equal tothe reduction rate of skin pass rolling.

[0136] Skin pass rolling and leveling are usually performed to adjustroughness and to correct shape. In the present invention, in additionthereto, skin pass rolling and leveling are effective in increasing andstabilizing the BH and ΔTS. Such an effect is remarkably caused at anelongation of 1.5% or more. However, if the elongation exceeds 10%,ductility is decreased. Therefore, working after hot-rolling ispreferably performed with an elongation of 1.5% to 10%. Additionally,although the working mode is different between skin pass rolling andleveling (the former is rolling and the latter is repeated bending andstretching), the effects of the elongation on the strain aginghardenability of the steel sheet of the present invention in bothworkings are substantially the same. In the present invention, acidpickling may be performed before or after the working after hot-rolling.

EXAMPLE 1

[0137] Each of the steels having the compositions shown in Table 1 wasmelted in a converter, and a slab was formed by continuous casting. Theslab was hot-rolled under the conditions shown in Table 2 to produce ahot-rolled steel sheet. In finish-rolling, sheet bars were not joined toeach other and tandem rolling was performed for the individual sheetbars. With respect to the resultant hot-rolled steel sheet, thedissolved N, the microstructure, the tensile characteristics, the strainaging hardenability, and improvements in fatigue resistance and impactresistance due to strain aging treatment were investigated.

[0138] The amount of dissolved N was measured by the method describedabove.

[0139] In order to observe the microstructure, with respect to the Ccross section (the cross section perpendicular to the rolling direction)excluding the portions 10% from the surfaces in the thickness direction,the enlarged image of the structure appearing due to corrosion wasanalyzed.

[0140] The tensile tests for checking the tensile characteristics andthe strain aging hardenability were performed according to JIS Z 2241using JIS No. 5 test pieces.

[0141] The strain aging treatment was performed with a prestrain of 5%under the aging treatment conditions: 170° C.×20 minutes.

[0142] The fatigue resistance was evaluated by the fatigue limitobtained by a tensile fatigue test according to JIS Z 2273.

[0143] The impact resistance was evaluated by the absorbed energy foundby integrating stress in the strain range of 0 to 30% with respect tothe stress-strain curve measured at a strain rate of 2,000/s accordingto a high-speed tensile test method described in “Journal of the Societyof Materials Science Japan. 47,10(1998)1058”.

[0144] The results thereof are shown in Table 3. In the examples of thepresent invention, significantly higher BH and ΔTS were observedcompared to the comparative examples, and the improvements in fatigueresistance and impact resistance due to the strain aging treatment werelarger compared to the comparative examples.

[0145] Additionally, the characteristics of plated steel sheets obtainedby hot-dip galvanizing the steel Nos. C and D were substantially thesame as those of the steel sheets before plating. In order to performplating treatment, the steel sheet was immersed in a galvanizing bathand after the immersed steel sheet was retrieved, the-areal weight wasadjusted by gas-wiping. The plating treatment was performed under theconditions of sheet temperature: 475° C., plating bath: 0.13% Al-Zn,bath temperature: 475° C., immersion time: 3 seconds, and areal weight:45 g/m².

EXAMPLE 2

[0146] The steel having the composition shown in Table 4 was cast into aslab in the same manner as Example 1, and the slab was hot-rolled underthe conditions shown in Table 5. Thereby, hot-rolled steel sheets (witha thickness of 1.6 mm) in which the average cooling rates were greatlyvaried were obtained. In such a case, when finish-rolling was performed,consecutive sheet bars with a thickness of 25 mm were joined to eachother by fusion-pressure welding at the initial stand, and tandemrolling was performed continuously. Between rough-rolling andfinish-rolling, the temperature of the sheet bar was adjusted using asheet bar edge heater and a sheet bar heater of induction heating type.The resultant hot-rolled steel sheets were investigated in the samemanner as Example 1.

[0147] The results thereof are shown in Table 6. In all the steelsheets, it is clear that the strain aging hardenability was stable at ahigh level. In Example 2, due to the continuous rolling and thetemperature adjustment of the sheet bar, the thickness accuracy and theshape were improved compared to Example 1. Furthermore, sincefinish-rolling was continuously performed by joining consecutive sheetbars to each other, the rolling conditions and cooling conditions forone sheet bar were uniformly set in the entire length in thelongitudinal direction. As a result, stable strain aging hardenabilitywas confirmed over the entire length of the steel sheet.

EXAMPLE 3

[0148] With respect to the steel sheet Nos. A, N, and J shown in Table3, the BH (increase in deformation stress) and the ΔTS (increase intensile strength) were investigated with varied aging treatmentconditions. The results thereof are shown in FIGS. 1 and 2. In theexamples of the present invention (A and N), significantly greaterhardening was observed compared to the comparative example (J) in thelow-temperature, short-time aging treatment. Thereby, it is obvious thatthe steel sheet of the present invention has superior strain aginghardenability. It is also clear that the examples A and N of the presentinvention exhibit superior strain aging hardenability under the strainaging treatment conditions in the wide ranges of 100 to 300° C.×30seconds to 20 minutes.

EXAMPLE 4

[0149] Each of the steels having the compositions shown in Tables 7 and8 was melted in a converter, and a slab was formed by continuouscasting. The slab was hot-rolled under the conditions shown in Tables 9and 10 to produce a hot-rolled steel sheet. With respect to theresultant hot-rolled steel sheet, the dissolved N, the microstructure,the tensile characteristics, strain aging hardenability, andimprovements in fatigue resistance and impact resistance due to strainaging treatment were investigated.

[0150] The amount of dissolved N was measured by the method describedabove.

[0151] In order to observe the microstructure, with respect to the Ccross section (the cross section perpendicular to the rolling direction)in the center in the thickness direction, the enlarged image of thestructure appearing due to corrosion was analyzed.

[0152] The tensile tests for checking the tensile characteristics andthe strain aging hardenability were performed according to JIS Z 2241using JIS No. 5 test pieces.

[0153] The strain aging treatment was performed with a prestrain of 5%under the aging treatment conditions: 170° C.×20 minutes.

[0154] The fatigue resistance and the impact resistance were evaluatedin the same manner as Example 1.

[0155] The results thereof are shown in Tables 11 and 12. In theexamples of the present invention, significantly higher BH and ΔTS wereobserved compared to the comparative examples, and the improvements infatigue resistance and impact resistance due to the strain agingtreatment were larger compared to the comparative examples.

[0156] Additionally, the characteristics of plated steel sheets obtainedby hot-dip galvanizing the steel Nos. C and D were substantially thesame as those of the steel sheets before plating. In order to performplating treatment, the steel sheet was immersed in a galvanizing bathand after the immersed steel sheet was retrieved, the areal weight wasadjusted by gas-wiping. The plating treatment was performed under theconditions of sheet temperature: 475° C., plating bath: 0.13% Al-Zn,bath temperature: 475° C., immersion time: 3 seconds, and areal weight45 g/m².

[0157] With respect to the steel sheet No. A (steel of the presentinvention) and the steel sheet No. 0 (comparative steel) shown in Tables11 and 12, BH and ΔTS were investigated with a prestrain of 5% under theaging treatment conditions shown in Table 13. Table 13 also shows theresults thereof.

[0158] As is obvious from Table 13, the steel No. A of the presentinvention exhibits high values of BH and ΔTS even under the relativelylow-temperature, short-time aging treatment conditions of 100° C.×30seconds.

EXAMPLE 5

[0159] Each of the steels having the compositions shown in Table 14 wasmelted in a converter, and a slab was formed by continuous casting. Theslab was hot-rolled under the conditions shown in Table 15 to produce ahot-rolled steel sheet. In finish-rolling, sheet bars were not joined toeach other and tandem rolling was performed for the individual sheetbars. With respect to the resultant hot-rolled steel sheet, thedissolved N, the microstructure, the tensile characteristics, the strainaging hardenability, and improvements in fatigue resistance and impactresistance due to strain aging treatment were investigated.

[0160] The amount of dissolved N, the amount of precipitated Nb*, andthe amount of precipitated V were measured by the methods describedabove.

[0161] In order to observe the microstructure, with respect to the Ccross section (the cross section perpendicular to the rolling direction)excluding the portions 10% from the surfaces in the thickness direction,the enlarged image of the structure appearing due to corrosion wasanalyzed. The average grain size of Nb and V carbonitrides was obtainedusing a transmission electron microscope and an energy-dispersive X-rayanalyzer.

[0162] The tensile tests for checking the tensile characteristics andthe strain aging hardenability were performed according to JIS Z 2241using JIS No. 5 test pieces.

[0163] The strain aging treatment was performed with a prestrain of 5%under the aging treatment conditions: 170° C.×20 minutes.

[0164] The fatigue resistance and the impact resistance were evaluatedby the methods described in Example 1. Furthermore, in order to evaluatethe impact resistance and the fatigue resistance relative to thestrength level of the steel sheet (strain aged steel), the ratio ofabsorbed energy En (MJ/) to the tensile strength TS (MPa) of the strainaged steel, En/TS (MJ/(MPa)) and the ratio of the fatigue limit σw (MPa)to the tensile strength TS (MPa) of the strain aged steel, σw/TS wereobtained.

[0165] The results thereof are shown in Table 16. In the examples of thepresent invention, the values of BH and ΔTS are large, and also highfatigue resistance and impact resistance are exhibited. The values ofEn/TS and σw/TS are also large, and superior fatigue resistance andimpact resistance are exhibited compared to the comparative steelshaving the same strength level.

[0166] Additionally, the characteristics of a plated steel sheetobtained by hot-dip galvanizing the steel sheet No. C1 weresubstantially the same as those of the steel sheet before plating. Inorder to perform plating treatment, the steel sheet was immersed in agalvanizing bath and after the immersed steel sheet was retrieved, theareal weight was adjusted by gas-wiping. The plating treatment wasperformed under the conditions of sheet temperature: 475° C., platingbath: 0.13% Al-Zn, bath temperature: 475° C., immersion time: 3 seconds,and areal weight 45 g/m².

INDUSTRIAL APPLICABILITY

[0167] With respect to the high tensile strength hot-rolled steel sheetof the present invention, since dissolved N is appropriately used, thestrength of the mother plate with a TS of 440 MPa or more is exhibited,and superior strain aging hardenability with a BH of 80 MPa or more anda ΔTS of 40 MPa or more is exhibited after strain aging treatment isperformed. The same characteristics are exhibited after plating isperformed, and moreover, it is possible to perform hot-rollinginexpensively without disturbing the shape. The thickness of the steelsheet used for automotive components can be decreased, for example, fromapproximately 2.0 mm to approximately 1.6 mm, thus greatly contributingto lightening of automobile bodies. TABLE 1 Steel C Si Mn P S Al NOthers No. % % % % % % % N/Al % 1 0.07 0.25 1.80 0.015 0.003 0.0120.0105 0.88 — 2 0.05 0.50 1.60 0.008 0.002 0.008 0.0150 1.88 — 3 0.080.15 2.00 0.010 0.002 0.011 0.0095 0.86 — 4 0.05 0.35 1.75 0.005 0.0020.011 0.0120 1.09 Mo:0.15 5 0.05 0.45 1.65 0.045 0.001 0.007 0.0123 1.76— 6 0.05 0.15 2.00 0.008 0.001 0.004 0.0140 3.50 Ti:0.015 7 0.03 0.152.00 0.008 0.001 0.011 0.0140 1.27 Nb:0.015,B:0.0008 8 0.05 0.15 1.550.004 0.003 0.011 0.0121 1.10 Ni:0.05 9 0.05 0.15 1.61 0.008 0.002 0.0050.0118 2.36 Cu:0.10,Ni:0.05 10 0.07 0.25 1.80 0.015 0.003 0.004 0.00420.08 — 11 0.05 0.15 1.80 0.007 0.002 0.004 0.0140 3.50 Cu:0.15 12 0.050.15 1.80 0.007 0.002 0.004 0.0145 3.63 V:0.015 13 0.05 0.15 1.77 0.0070.002 0.004 0.0142 3.55 Cr:0.15,Ti:0.015 14 0.06 0.15 1.78 0.005 0.0020.004 0.0141 3.53 Nb:0.015,V:0.015 15 0.04 0.15 1.82 0.004 0.002 0.0040.0139 3.48 Ni:0.05,Ti:0.015 16 0.05 0.15 1.81 0.005 0.002 0.004 0.01413.53 Cu:0.10,B:0.003 17 0.05 0.15 1.80 0.007 0.002 0.004 0.0140 3.50Ca:0.0015 18 0.04 0.15 1.78 0.007 0.002 0.004 0.0141 3.53Cu:0.10,Ca:0.002 19 0.05 0.15 1.77 0.005 0.002 0.004 0.0140 3.53Nb:0.020,REM:0.002 20 0.05 0.15 1.81 0.006 0.002 0.004 0.0140 3.50B:0.0003 21 0.05 0.15 1.80 0.007 0.002 0.004 0.0140 3.50B:0.0002,REM:0.002 22 0.04 0.15 1.79 0.007 0.002 0.004 0.0141 3.53Cr:0.10,Nb:0.02 B:0.0003,Ca:0.0015 23 0.08 0.15 2.00 0.010 0.002 0.0160.0050 0.31 —

[0168] TABLE 2 Steel Thick- Sheet Steel. SRT FDT ness Δt V CT No. No. °C. ° C. mm s ° C./s ° C. Others A 1 1,220 880 1.6 0.2 80 520 — B 2 1,200890 1.8 0.2 65 540 — C 3 1,150 890 1.4 0.1 75 520 — D 4 1,220 850 1.60.1 75 570 — E 5 1,270 850 1.8 0.2 65 580 — F 6 1,200 890 1.8 0.3 65 520— G 7 1,100 840 2.3 0.2 55 530 — H 8 1,100 845 2.0 0.3 60 540 — I 91,100 850 1.8 0.4 70 530 HCR J 10 1,100 880 1.8 0.3 70 530 — K 1 1,130840 1.8 1.5 70 540 — L 1 1,220 850 1.8 0.3 70 680 — M 1 1,220 880 1.80.2 70 600 — N 1 1,220 890 1.8 0.2 70 250 LV O 1 1,230 880 1.4 0.2 73420 SK P 11 1,200 890 1.8 0.3 65 530 — Q 12 1,200 890 1.8 0.3 65 530 — R13 1,200 890 1.8 0.3 65 530 — S 14 1,200 890 1.8 0.3 65 530 — T 15 1,200890 1.8 0.3 65 530 — U 16 1,200 890 1.8 0.3 65 530 — V 17 1,200 890 1.80.3 65 530 — W 18 1,200 890 1.8 0.3 65 530 — X 19 1,200 890 1.8 0.3 65530 — Y 20 1,200 890 1.8 0.3 65 530 — Z 21 1,200 890 1.8 0.3 65 530 — AA22 1,200 890 1.8 0.3 65 530 — AB 23 1,150 890 1.4 0.5 40 646 —

[0169] TABLE 3 Remarks (PI: Steel sheet Strain Example of Dis- Steelsheet tensile aging present solved N structure character- harden-Fatigue invention Steel in steel Phase istics ability resis- Impact CE:Sheet sheet compo- Vα d YS TS El BH ΔTS tance resis- Comparative No. %sition % μm MPa MPa % MPa MPa MPa tance example) A 0.0071 F,P,B 85 8.2351 474 38 113 55 95 1.18 PI B 0.0121 F,P,B 90 8.4 368 469 36 110 52 901.15 PI C 0.0060 F,B 85 7.9 355 512 35 115 61 97 1.19 PI D 0.0082 F,B 877.8 365 532 34 115 63 98 1.18 PI E 0.0112 F,P,B 92 8.1 338 485 37 108 5594 1.16 PI F 0.0075 F,B 85 7.4 353 508 36 92 62 98 1.19 PI G 0.0088 F,B83 5.9 411 610 31 112 74 101 1.19 PI H 0.0084 F,P 93 7.8 326 465 37 10852 88 1.15 PI I 0.0102 F,B 88 8.3 331 475 38 105 55 89 1.13 PI J 0.0002F,P,B 85 8.4 334 454 37 22 5 0 1.00 CE K 0.0008 F,P,B 90 10.8 332 434 3832 15 20 1.01 CE L 0.0005 F,P 95 11.0 295 411 38 10 12 18 0.99 CE M0.0065 R,P,B 86 8.3 348 468 38 110 50 93 1.13 PI N 0.0100 F,M 83 7.9 363605 34 155 105 125 1.25 PI O 0.0105 F,M,B 86 7.6 355 481 37 118 63 1121.20 PI P 0.0095 F,B 85 7.7 361 485 38 120 69 105 1.21 PI Q 0.0093 F,B87 7.4 371 480 36 118 59 98 1.18 PI R 0.0082 F,B,M 82 6.5 365 505 38 11971 102 1.18 PI S 0.0075 F,B 82 6.3 381 485 37 119 69 103 1.20 PI T0.0085 F,B 85 6.5 359 479 38 115 56 99 1.19 PI U 0.0072 F,B 84 7.2 358480 38 115 57 98 1.18 PI V 0.0098 F,B 85 8.1 355 475 39 102 65 101 1.19PI W 0.0101 F,B 83 8.0 365 480 38 113 69 104 1.18 PI X 0.0095 F,B 81 5.9480 510 36 119 75 102 1.19 PI Y 0.0120 F,B 85 7.1 355 475 39 115 59 991.19 PI Z 0.0115 F,B 85 7.2 360 479 38 115 61 102 1.18 PI AA 0.0115 F,B82 5.8 369 525 37 118 65 109 1.19 PI AB 0.0011 F,P,B 85 9.5 368 471 3699 53 88 1.18 PI

[0170] TABLE 4 Oth- Steel C Si Mn P S Al N ers No. % % % % % % % N/Al %24 0.08 0.35 1.55 0.009 0.002 0.012 00135 1.11 —

[0171] TABLE 5 Steel Thick- Sheet Steel SRT FDT ness Δt V CT Oth- Re-No. No. ° C. ° C. mm s ° C./s ° C. ers marks AC 11 1,280 920 1.6 0.2 95480 Con- PI tin- uous roll- ing AD 11 1,220 890 1.6 0.2 65 520 Con- PItin- uous roll- ing AE 11 1,180 925 1.6 0.1 100 520 Con- PI tin- uousroll- ing

[0172] TABLE 6 Remarks (PI: Steel sheet Strain Example of Dis- Steelsheet tensile aging present solved N structure character- harden-Fatigue invention Steel in steel Phase istics ability resis- Impact CE:Sheet sheet compo- Vα d YS TS El BH ΔTS tance resis- Comparative No. %sition % μm MPa MPa % MPa MPa MPa tance example) AC 0.0095 F,P,B 88 8.1351 474 38 115 58 95 1.19 PI AD 0.0092 F,P,B 89 8.3 368 469 37 110 52 901.15 PI AE 0.0088 F,P,B 85 7.6 364 495 37 115 65 100 1.18 PI

[0173] TABLE 7 Steel C Si Mn P S Al N Others No. % % % % % % % N/Al % 10.07 0.25 1.80 0.015 0.003 0.012 0.0105 0.88 — 2 0.05 0.50 1.60 0.0080.002 0.008 0.0150 1.88 — 3 0.08 0.15 2.00 0.010 0.002 0.011 0.0095 0.86— 4 0.05 0.35 1.75 0.005 0.002 0.011 0.0120 1.09 Mo:0.15 5 0.05 0.451.65 0.045 0.001 0.007 0.0123 1.76 — 6 0.05 0.15 2.00 0.008 0.001 0.0040.0140 3.50 Ti:0.015 7 0.03 0.15 2.00 0.008 0.001 0.011 0.0140 1.27Nb:0.015,B:0.0008 8 0.05 0.15 1.55 0.004 0.003 0.011 0.0121 1.10 Ni:0.059 0.05 0.15 1.61 0.008 0.002 0.005 0.0118 2.36 Cu:0.10,Ni:0.05 10 0.070.25 1.80 0.015 0.003 0.055 0.0042 0.08 — 11 0.08 0.35 1.55 0.009 0.0020.012 0.0135 1.12 Mo:0.50 12 0.05 0.15 1.80 0.007 0.002 0.004 0.01403.50 Cu:0.15

[0174] TABLE 8 Steel C Si Mn P S Al N Others No. % % % % % % % N/Al % 130.05 0.15 1.80 0.007 0.002 0.004 0.0145 3.63 V:0.015 14 0.05 0.15 1.770.007 0.002 0.004 0.0142 3.55 Cr:0.15,Ti:0.015 15 0.06 0.15 1.78 0.0050.002 0.004 0.0141 3.53 Nb:0.015,V:0.015 16 0.04 0.15 1.82 0.004 0.0020.004 0.0139 3.48 Ni:0.05,Ti:0.015 17 0.05 0.15 1.81 0.005 0.002 0.0040.0141 3.53 Cu:0.10,B:0.0030 18 0.05 0.15 1.80 0.007 0.002 0.004 0.01403.50 Ca:0.0015 19 0.04 0.15 1.78 0.007 0.002 0.004 0.0141 3.53Cu:0.10,Ca:0.0020 20 0.05 0.15 1.77 0.005 0.002 0.004 0.0140 3.53Nb:0.020,REM:0.0020 21 0.05 0.15 1.81 0.006 0.002 0.004 0.0140 3.50B:0.0003 22 0.05 0.15 1.80 0.007 0.002 0.004 0.0140 3.50B:0.0002,REM:0.0020 23 0.04 0.15 1.79 0.007 0.002 0.004 0.0141 3.53Cr:0.10,Nb:0.02 B:0.003,Ca:0.0015 24 0.08 0.15 2.00 0.010 0.002 0.0160.0050 0.31 — 25 0.06 0.15 2.65 0.015 0.002 0.012 0.0142 1.18Nb:0.008,Ti:0.005 26 0.08 0.15 2.95 0.015 0.002 0.005 0.0180 3.60 — 270.08 0.45 2.90 0.011 0.002 0.011 0.0175 1.59 Nb:0.038

[0175] TABLE 9 Steel Thick- Sheet Steel. SRT FDT ness Δt V CT No. No. °C. ° C. mm s ° C./s ° C. Others A 1 1,180 880 2.3 0.3 55 280 — B 2 1,180880 2.3 0.3 55 400 — C 3 1,170 880 2.3 0.3 55 380 — D 4 1,200 890 1.60.3 60 380 — E 5 1,220 890 1.6 0.3 60 400 JCR F 6 1,200 890 1.6 0.3 60325 — G 7 1,220 870 1.6 0.3 60 280 — H 8 1,270 870 1.6 0.3 60 250 — I 91,250 850 1.8 0.2 60 320 HCR J 10 1,250 850 1.8 0.2 60 350 — K 1 1,270850 1.8 0.2 60 350 — L 1 1,250 850 1.4 0.2 70 290 LV M 1 1,250 850 1.40.2 70 320 — N 1 1,250 850 1.4 0.2 70 560 — O 1 950 720 1.4 0.2 70 350 —P 11 1,180 880 2.0 0.2 50 350 SK

[0176] TABLE 10 Steel Thick- Sheet Steel SRT FDT ness Δt V CT No. No. °C. ° C. mm s ° C./s ° C. Others Q 11 1,180 880 2.0 2.0 55 360 — R 111,180 880 2.0 0.2 10 350 — S 12 1,200 885 1.6 0.3 55 250 — T 13 1,220890 1.6 0.3 60 350 — U 14 1,220 900 1.6 0.2 55 300 — V 15 1,220 885 1.60.3 55 300 — W 16 1,200 895 1.6 0.3 55 300 — X 17 1,200 890 1.6 0.3 55280 — Y 18 1,220 900 1.6 0.3 60 250 — Z 19 1,200 905 1.6 0.3 55 280 — AA20 1,220 910 1.6 0.3 50 250 — AB 21 1,180 910 1.6 0.2 55 250 — AC 221,180 910 1.6 0.3 60 280 — AD 23 1,200 900 1.6 0.2 65 250 — AE 24 1,210890 1.6 0.4 40 320 — AF 25 1,170 870 1.6 0.4 45 380 — AG 26 1,200 8901.6 0.4 85 400 — AH 27 1,250 910 1.6 0.3 65 420 —

[0177] TABLE 11 Remarks (PI: Example of Dis- Strain aging present solvedN Steel sheet structure Steel sheet tensile harden- Fatigue inventionSteel in steel Phase characteristics ability resis- Impact CE: Sheetsheet compo- Vα d VM YS TS El BH ΔTS tance resis- Comparative No. %sition % μm % MPa MPa YR % MPa MPa MPa tance example) A 0.0080 F,M,B 816.9 17 403 620 0.65 32 151 85 125 1.29 PI B 0.0120 F,M,B 87 6.9 12 385598 0.64 33 150 95 119 1.28 PI C 0.0072 F,M 79 5.7 21 415 645 0.64 30165 90 118 1.28 PI D 0.0097 F,M 82 6.8 18 402 625 0.64 31 150 101 1211.31 PI E 0.0105 F,M,B 86 6.8 12 395 605 0.65 31 150 92 115 1.28 PI F0.0110 F,M 79 6.1 21 420 650 0.65 29 161 90 122 1.27 PI G 0.0085 F,M 896.7 11 367 565 0.65 34 150 102 119 1.29 PI H 0.0095 F,M,B 86 6.8 12 370570 0.65 33 151 88 125 1.28 PI I 0.0085 F,M,B 85 6.6 14 391 605 0.65 32155 105 115 1.31 PI J 0.0008 F,M,B 81 6.9 13 385 595 0.65 28 75 42 451.10 CE K 0.0085 F,M,B 82 6.9 16 401 620 0.65 31 159 87 115 1.27 PI L0.0087 F,M 83 6.6 17 420 630 0.67 31 160 85 120 1.28 PI M 0.0087 F,M 836.6 17 405 620 0.65 32 150 92 115 1.29 PI N 0.0085 F,P,B 90 8.0 0 415530 0.78 29 72 15 51 1.09 CE O 0.0045 F,B,M 97 10.9 3 395 505 0.78 34 4010 57 1.08 CE P 0.0082 F,M 85 6.8 15 342 598 0.57 32 145 88 115 1.27 PI

[0178] TABLE 12 Remarks (PI: Example of Dis- Strain aging present solvedN Steel sheet structure Steel sheet tensile harden- Fatigue inventionSteel in steel Phase characteristics ability resis- Impact CE: Sheetsheet compo- Vα d VM YS TS El BH ΔTS tance resis- Comparative No. %sition % μm % MPa MPa YR % MPa MPa MPa tance example) Q 0.0042 F,P,B,M95 10.5 3 392 520 0.78 33 70 15 53 1.08 CE R 0.0032 F,P,B,M 97 10.5 2406 520 0.78 33 65 18 55 1.09 CE S 0.0115 F,M 82 6.7 18 404 628 0.64 31152 102 122 1.30 PI T 0.0125 F,M,B 83 6.8 16 400 630 0.63 31 138 105 1181.29 PI U 0.0110 F,M 82 6.6 18 415 640 0.67 31 152 105 120 1.31 PI V0.0120 F,M 84 5.9 16 410 645 0.63 30 155 105 125 1.30 PI W 0.0105 F,M 846.4 16 395 625 0.63 31 145 102 120 1.28 PI X 0.0105 F,M 83 6.4 17 390615 0.63 32 140 95 105 1.25 PI Y 0.0120 F,M 84 6.2 16 370 615 0.60 31150 98 110 1.28 PI Z 0.0115 F,M 85 6.1 16 365 619 0.58 31 155 102 1151.25 PI AA 0.0120 F,M 85 5.2 15 445 649 0.68 31 168 95 125 1.32 PI AB0.0120 F,M 82 6.7 18 385 620 0.62 32 151 105 115 1.28 PI AC 0.0110 F,M83 6.8 17 380 620 0.61 32 145 105 110 1.25 PI AD 0.0105 F,M 80 6.4 20405 669 0.60 30 140 108 105 1.24 PI AE 0.0010 F,M,B 87 6.9 10 365 5950.61 33 105 72 95 1.18 PI AF 0.0086 F,M 49 7.0 51 540 795 0.68 19 95 7195 1.15 PI AG 0.0135 F,M,B 45 5.1 42 600 997 0.60 14 153 102 98 1.05 PIAH 0.0131 F,B,M 45 5.3 12 650 1,080   0.60 13 145 98 94 1.06 PI

[0179] TABLE 13 Aging treatment conditions A (Steel of present Heat-Heat- invention) O (Comparative steel) treating treating BH ΔTS BH ΔTStemperature time (MPa) (MPa) (MPa) (MPa) 100° C. 30 sec 120 60 20 3 100°C. 10 min 130 70 24 3 100° C. 20 min 135 75 25 4 300° C. 30 sec 140 6530 5 300° C. 10 min 155 70 35 5 300° C. 20 min 160 70 40 10  170° C. 20min 151 85 40 10 

[0180] TABLE 14 Steel C Si Mn P S Al N Nb V N/Al No. % % % % % % % % % —A 0.06 0.02 1.2 0.012 0.0030 0.015 0.015 0.2 — 1.0 B 0.08 0.02 1.0 0.0100.0050 0.015 0.015 0.040 — 1.0 C 0.05 0.02 1.4 0.010 0.0040 0.012 0.0150.070 — 1.25 D 0.08 0.4 1.7 0.015 0.0040 0.015 0.015 0.050 — 1.0 E 0.050.2 1.2 0.010 0.0050 0.011 0.015 0.010 — 1.36 F 0.04 0.1 1.3 0.0120.0030 0.015 0.017 — 0.15 1.13 G 0.08 0.02 1.4 0.015 0.0040 0.015 0.015— 0.05 1.0 H 0.06 0.7 0.9 0.010 0.0030 0.017 0.020 — 0.08 1.18 I 0.080.8 1.8 0.007 0.0020 0.004 0.014 —  0.010 3.5 J 0.05 0.1 1.2 0.0100.0040 0.010 0.018 0.03 0.03 1.8 K 0.03 0.2 1.8 0.010 0.0030 0.0120.0010 0.04 — 0.08 L 0.06 0.01 1.5 0.015 0.0050 0.010 0.004 — 0.05 0.4

[0181] TABLE 15 Steel Thick- Steel sheet SRT FDT ness Δt V CT No. No. °C. ° C. mm s ° C./s ° C. A A1 1,220 820 1.6 0.2 50 600 B B1 1,250 8501.8 0.1 50 550 B2 1,250 850 1.8 0.1 50 700 B3 1,250 850 1.8 0.1 50 450B4 1,050 850 1.8 0.1 50 600 C C1 1,250 880 1.4 0.1 80 550 D D1 1,220 8802.9 0.3 50 600 E E1 1,220 850 1.8 0.2 50 600 F F1 1,250 850 1.6 0.2 60640 G G1 1,220 850 1.4 0.1 100  550 G2 1,220 850 1.4 0.1 100  720 G31,220 850 1.4 0.1 100  450 G4 1,220 850 1.4 1.0 100  600 H H1 1,250 8802.3 0.2 50 600 I I1 1,250 850 1.6 0.2 50 540 J J1 1,230 880 2.0 0.2 50560 J2 1,250 880 2.0 0.2 10 640 K K1 1,250 880 1.8 0.1 60 580 L L1 1,250850 1.6 0.3 50 600

[0182] TABLE 16 Remarks (PI: Dis- Example of solved Steel Strain agingpresent N in sheet Steel sheet structure Steel sheet Tensile harden-Fatigue invention Steel steel Nb* + Phase characteristics ability resis-Impact CE: Sheet sheet V* compo- Vα d dp YS TS El BH ΔTS tance resis-Comparative No. % % sition % μm μm MPa MPa % MPa MPa MPa tance En/TSσ_(w)/TS example) A1 0.0020 0.080 F + B 92 10.2 0.5 405 581 26 82 35 351.02 0.29 0.82 CE B1 0.0120 0.032 F + B 90 6.8 0.03 515 624 28 88 46 1031.19 0.33 1.05 PI B2 0.0009 0.038 F + B 97 12.4 0.19 402 583 29 32 8 181.04 0.28 0.80 CE B3 0.0140 0.008 F + B 78 6.2 0.02 467 649 24 91 42 1061.22 0.30 0.96 CE B4 0.0015 0.031 F + B 95 9.8 0.8 410 592 25 81 40 881.13 0.29 0.98 CE C1 0.0142 0.057 F + B 92 6.8 0.03 515 617 27 84 44 1051.18 0.34 1.03 PI D1 0.0090 0.041 F + P + B 82 5.9 0.02 652 804 19 87 4295 1.15 0.33 1.07 PI E1 0.0092 0.008 F + B 94 6.5 0.03 390 566 30 88 4299 1.16 0.31 0.90 CE F1 0.0030 0.071 F + B 92 11.8 0.3 451 610 24 81 2038 1.03 0.30 0.84 CE G1 0.0125 0.041 F + B 95 6.9 0.02 521 622 27 84 44102 1.18 0.34 1.07 PI G2 0.0008 0.045 F + B 98 11.6 0.28 392 571 29 25 421 1.02 0.29 0.79 CE G3 0.0139 0.009 F + B 82 5.5 0.02 450 655 22 87 42104 1.19 0.30 0.94 CE G4 0.0009 0.030 F + B 94 10.3 0.04 396 577 29 31 519 1.01 0.29 0.81 CE H1 0.0182 0.060 F + B 90 5.9 0.02 655 811 18 88 4098 1.15 0.33 1.04 PI I1 0.0125 0.009 F + P + B 85 6.2 0.02 559 804 17 8142 100 1.18 0.31 0.96 CE J1 0.0155 0.048 F + B 92 6.8 0.02 529 621 28 8445 102 1.21 0.34 1.06 PI J2 0.0008 0.021 F + B 97 10.9 0.07 381 560 2927 7 24 1.02 0.30 0.83 CE K1 0.0002 0.017 F + B 90 6.2 0.03 467 599 2811 2 19 1.00 0.29 0.80 CE L1 0.0009 0.026 F + B 93 6.9 0.04 472 602 2920 5 21 1.04 0.29 0.81 CE

1. A high tensile strength hot-rolled steel sheet having superior strainaging hardenability comprising: in percent by mass, 0.15% or less of C;2.0% or less of Si; 3.0% or less of Mn; 0.08% or less of P; 0.02% orless of S; 0.02% or less of Al; 0.0050% to 0.0250% of N; and the balancebeing Fe and incidental impurities, the ratio N (mass %)/Al (mass %)being 0.3 or more, N in the dissolved state being 0.0010% or more.
 2. Ahigh tensile strength hot-rolled steel sheet having superior strainaging hardenability with a tensile strength of 440 MPa or morecomprising: in percent by mass, 0.15% or less of C; 2.0% or less of Si;3.0% or less of Mn; 0.08% or less of P; 0.02% or less of S; 0.02% orless of Al; 0.0050% to 0.0250% of N; and the balance being Fe andincidental impurities, the ratio N (mass %)/Al (mass %) being 0.3 ormore, N in the dissolved state being 0.0010% or more, wherein thehot-rolled steel sheet has a structure in which the areal rate of theferrite phase having an average grain size of 10 μm or less is 50% ormore.
 3. A steel sheet according to claim 2 further comprising at leastone selected from the group consisting of the following Group a to Groupd: Group a: 1.0% or less in total of at least one of Cu, Ni, Cr, and MoGroup b: 0.1% or less in total of at least one of Nb, Ti, and V Group c:0.0030% or less of B Group d: 0.0010% to 0.010% in total of at least oneof Ca and REM.
 4. A steel sheet according to either claim 2 or 3,wherein the high tensile strength hot-rolled sheet has a thickness of4.0 mm or less.
 5. A high tensile strength hot-rolled plated steel sheetproduced by electroplating or hot-dip plating a steel sheet according toany one of claims 2 to
 4. 6. A method for producing a high tensilestrength hot-rolled steel sheet having superior strain aginghardenability with a tensile strength of 440 MPa or more comprising thesteps of: heating a steel slab to 1,000° C. or more, the steel slabcomprising: in percent by mass, 0.15% or less of C; 2.0% or less of Si;3.0% or less of Mn; 0.08% or less of P; 0.02% or less of S; 0.02% orless of Al; 0.0050% to 0.0250% of N; and optionally further comprisingat least one selected from the group consisting of the following Group ato Group d, the ratio N (mass %)/Al (mass %) being 0.3 or more: Group a:1.0% or less in total of at least one of Cu, Ni, Cr, and Mo Group b:0.1% or less in total of at least one of Nb, Ti, and V Group c: 0.0030%or less of B Group d: 0.0010% to 0.010% in total of at least one of Caand REM; rough-rolling the steel slab to form a sheet bar;finish-rolling the sheet bar at a finishing temperature of 800° C. ormore; cooling at a cooling rate of 20° C./s or more within 0.5 secondafter the finish-rolling; and coiling at a temperature of 650° C. orless.
 7. A method according to according to claim 6, further comprisingthe step of performing at least one of skin pass rolling and levelingwith an elongation of 1.5% to 10% after the coiling step is performed.8. A method according to either claim 6 or 7, further comprising thestep of joining consecutive sheet bars to each other between the stepsof rough-rolling and finish-rolling.
 9. A method according to any one ofclaims 6 to 8, further comprising the step of using at least one of asheet bar edge heater for heating a widthwise end of the sheet bar and asheet bar heater for heating a lengthwise end of the sheet bar betweenthe steps of rough-rolling and finish-rolling.
 10. A high tensilestrength hot-rolled steel sheet having superior strain aginghardenability with a BH of 80 MPa or more, a ΔTS of 40 MPa or more, anda tensile strength of 440 MPa or more comprising, in percent by mass,0.15% or less of C; 2.0% or less of Si; 3.0% or less of Mn; 0.08% orless of P; 0.02% or less of S; 0.02% or less of Al; 0.0050% to 0.0250%of N; and the balance being Fe and incidental impurities, the ratio N(mass %)/Al (mass %) being 0.3 or more, N in the dissolved state being0.0010% or more, wherein the hot-rolled steel sheet has a structure inwhich the areal rate of the ferrite phase having an average grain sizeof 10 μm or less is 70% or more, and the areal rate of the martensitephase is 5% or more.
 11. A method for producing a high tensile strengthhot-rolled steel sheet having superior strain aging hardenability with aBH of 80 MPa or more, a ΔTS of 40 MPa or more, and a tensile strength of440 MPa or more comprising the steps of: heating a steel slab to 1,000°C. or more, the steel slab comprising: in percent by mass, 0.15% or lessof C; 2.0% or less of Si; 3.0% or less of Mn; 0.08% or less of P; 0.02%or less of S; 0.02% or less of Al; 0.0050% to 0.0250% of N; andoptionally further comprising at least one selected from the groupconsisting of the following Group a to Group d, the ratio N (mass %)/Al(mass %) being 0.3 or more: Group a: 1.0% or less in total of at leastone of Cu, Ni, Cr, and Mo Group b: 0.1% or less in total of at least oneof Nb, Ti, and V Group c: 0.0030% or less of B Group d: 0.0010% to0.010% in total of at least one of Ca and REM; rough-rolling the steelslab to form a sheet bar; finish-rolling the sheet bar at a finishingtemperature of 800° C. or more; cooling at a cooling rate of 20° C./s ormore within 0.5 second after the finish-rolling; and coiling at atemperature of 450° C. or less.
 12. A high tensile strength hot-rolledsteel sheet having superior strain aging hardenability comprising: inpercent by mass, 0.03% to 0.1% of C; 2.0% or less of Si; 1.0% to 3.0% ofMn; 0.08% or less of P; 0.02% or less of S; 0.02% or less of Al; 0.0050%to 0.0250% of N; 0.1% or less in total of at least one of more than0.02% to 0.1% of Nb and more than 0.02% to 0.1% of V; and the balancebeing Fe and incidental impurities, the ratio N (mass %)/Al (mass %)being 0.3 or more, N in the dissolved state being 0.0010% or more, thetotal of precipitated Nb and precipitated V being 0.015% or more,wherein the hot-rolled steel sheet has a structure in which the arealrate of the ferrite phase having an average grain size of 10 μm or lessis 80% or more, and the average grain size of a precipitate comprising aNb carbonitride or a V carbonitride is 0.05 μm or less.
 13. A method forproducing a high tensile strength hot-rolled steel sheet having superiorstrain aging hardenability comprising the steps of: heating a steel slabto 1,100° C. or more, the steel slab comprising: in percent by mass,0.03% to 0.1% of C; 2.0% or less of Si; 1.0% to 3.0% of Mn; 0.08% orless of P; 0.02% or less of S; 0.02% or less of Al; 0.0050% to 0.0250%of N: 0.1% or less in total of at least one of more than 0.02% to 0.1%of Nb and more than 0.02% to 0.1% of V; and the balance being Fe andincidental impurities; rough-rolling the steel slab to form a sheet bar;finish-rolling the sheet bar at a finishing temperature of 800° C. ormore; cooling at a cooling rate of 40° C./s or more within 0.5 secondafter the finish-rolling; and coiling in the temperature range of 550 to650° C.